I think these photos express the feeling best. For first-time visitors from the northern hemisphere, it can be a real shock because it's scorching hot, its laughter and fun, it's family/friends, it's a BBQ, it's beach, it's cricket, it's beer ...  lots of ice cold beer and it's singing christmas carols. Yes we sing White Christmas, Rudolph, Frosty the Snowman, Winter Wonderland, Let It Snow and other stuff that has nothing to do with our Southern Hemisphere climate ... but we sing them with great gusto anyway at the kids event "Carols in the Domain" in Sydney and at the "Myer Music Bowl" in Melbourne which is telecast to all Australians on National TV every Christmas eve.

BBQ - the typical Christmas lunch/afternoon event. Often by the pool or some waterway. Often just wearing thongs (flip-flops) ... on our feet!

Beach - Australian's head there at Christmas time because 80% of us live within just 50 kilometers of one and it's the coolest place to be on a scorching hot Christmas day.

Family Christmas lunch - Typically held outdoors because of the heat and at one long table where the extended family can gather and sit. Often with a 'hot' roast turkey and 'hot' Christmas pudding but washed down with plenty of ice cold beer and good cheer.
Beer - The great Aussie Christmas indulgence. Drinking too much beer at Christmas is an Aussie tradition. It is often given as a present with a 'slab' (carton) or a '6-pack' being the entry price for joining in the afternoon festivities with friends and family. What is not drunk on Christmas day is chilled in the fridge ready for the army of 'coach potatoes' who will spend the entire next day glued to the TV watching the Boxing Day test (cricket) and drinking the fridge dry.

Prawns - Ben Reimers reminds me that with the hot Aussie climate around Christmas, it makes it ideal for a cold delicacy that is usually beyond the family's weekly budget ... prawns. Australians spend 10 times more on prawns in the Christmas week than they do on average through out the year. It is the last minute purchase that causes the intense crush of crowds at fish mongers and supermarkets on the day before Christmas. Interestingly, one major retailer is getting in early with a prawn promotion that identifies how the humble prawn takes center stage on Christmas day down-under.

Cricket - Played at the beach, backyard, oval, park, street ... anywhere. After the Christmas lunch someone gets out a bat & ball and finds a rubbish tin or a couple of sticks to make a rudimentary cricket wicket. Everyone is expected to join in playing Australia's national sport ... including granddad, Aunt Mary, Santa Claus and any kids old enough to walk.
Sydney's Carols in the Domain .... for the kids.
Melbourne's Myer Music Bowl, the popular National telecast that every household tunes into and watches on Christmas eve.